This invention pertains to stepping motors and, more particularly, to methods of stopping stepping motors to minimize oscillations.
In many applications utilizing stepping motors, it is quite often necessary to rapidly and precisely stop the motor. A particular example is when the motor drives a printing head via a lead screw. While the drive is being performed, the rotor and the load coupled thereto possess considerable kinetic energy. Since the field generated by the stator windings is elastically coupled to the rotor, any rapid stopping of the rotation of the stator field results in the oscillation of the rotor and load about a rest position. This oscillation continues until the kinetic energy is expended--generally by friction within the system. Since a properly designed system minimizes friction, the oscillation can continue for a considerable time. If this oscillation is of any magnitude, then the stopping cannot be considered as occurring until the oscillation falls below a certain minimum value. In order to minimize such oscillations upon stoppping, there have been proposed controlled stopping routines for the the motor. Various stepping methods have been proposed. For example, there is a lengthy discussion in the article entitled "Control Aspects of Step Motors" by B. C. Kuo and R. A. Yackel pp. B-1 to B-23 (See especially text starting at line 3 from bottom of right-hand column of page B-21 to end of text) of PROCEEDINGS, SECOND ANNUAL SYMPOSIUM MOTION CONTROL SYSTEMS AND DEVICES, 1973 University of Illinois, Urbana-Champaign. Another such routine is discussed in "Dynamic Performance of a Three-Phase Variable-Reluctance Motor" by J. P. Pawletho and H. D. Chai, Pp. 1-6, of PROCEEDINGS SEVENTH ANNUAL SYMPOSIUM INCREMENTAL MOTOR CONTROL SYSTEMS AND DEVICES 1978, University of Illinois, Urbana-Champaign.
In the system disclosed in the second article, the acceleration and deceleration are regulated by controlling the phasing of the energization. In particular, during deceleration the driving pulses for stepping the motor are delayed in phase by a fixed amount. Furthermore, there is a programmed fixed number of such injected pulses. At the end of these injected pulses, another pulse is injected whose delay is also a programmed amount. Such a system works well for the drive of a constant known load such as the daisy wheel of a printer. However, when the load varies and when the tolerances imposed on the driving mechanism are reduced so as to reduce cost, such a routine cannot be used because for each different load or for each different motor there will be a different number of deceleration pulses as well as a different delay for the last pulse.
There is also described in Sec 11.3, entitled Description of the Variable-Unit Time-Delay Speed Control Systems, of STEP MOTORS AND CONTROL SYSTEMS, edited by Benjamin C. Kuo and published by SRL Publishing Company, Champaign, Ill. in 1979 a variable control system which controls the timing of the stepping pulses as a function of the difference between a desired velocity and the instantaneous velocity of rotation of the motor. Such control is an improvement of the system of the second article. However, when very precise control of the velocity to within a very narrow range before braking is desired to insure a precise stopping position other refinements must be employed.